An unmarked noun could be either singular or plural. Context usually makes it clear which is intended. Duolingo lacks many of the usual context clues (like whether we were already talking about one woman or many). Sometimes there are some clues within the sentence, like prefixes that are only singular or only plural, for instance. But because clarifying context is often missing, this Duolingo course often accepts either the singular or the plural.
The is no way to mark a noun as being exclusively singular, unless you want to count putting wa' (the number "one") in front of it. Usually you can only be certain it's just one based on context.
The plural suffixes can be used to mark a noun as being specifically plural. -pu' is the plural marker for beings capable of language. Since be' in this sentence has the plural marker -pu', it must be plural and cannot be singular.
An explanation is given in the Tips & Notes, but I know it’s a lot of new information and some details are bound to slip through the memory. However, since Duolingo has hidden the Tips & Notes I want to make sure you know about them and where to find them. If you have not been reading the Tips & Notes, I would like to ask that you review those so we don’t have to repeat too much of the information that we have explained there.
If you are doing the course on iOS or Android, you cannot currently access the Tips & Notes through the app. To access the Tips & Notes, you will have to access the course using a web browser at https://www.duolingo.com/. You can still do it on your mobile device, but you will have to use the web browser instead of the app (or you can do it from a computer). When you click on a Skill, it will expand to reveal a Start button, a key, and a light bulb.
If you click on the light bulb it will reveal the Tips & Notes and give you a detailed explanation of the grammar that is introduced in that Skill. If you have questions after reading the Tips & Notes for any Skills, then please return to the forum to ask your question, explaining what you didn’t understand or what seems contradictory to you.